The trend in electronics has been to reduce the size of circuits; a trend that culminated in the development of integrated circuits. While it is relatively simple to greatly reduce the dimensions of resistors and capacitors, unfortunately, it remains impractical to achieve a comparable reduction in the sizes of inductors. Accordingly, active elements (e.g., operational amplifiers or transconductance amplifiers configured to simulate inductive reactance) are often used as a substitute for inductors since they can be arranged to closely simulate an inductor's performance.
Many filter topologies require the use of coupled tuned circuits. For those designs that require response symmetry, some form of inductive coupling must be used. Regrettably, inductors formed from looped or coiled wires require a high number of turns to achieve the appropriate circuit response. Therefore to use such inductors human operators are required to tune each circuit for an appropriate response. Such an undertaking is very time consuming and costly to the consumers. Additionally, wire-wound inductors often lose their tuned response with age, and as a consequence, these inductors have to be re-tuned to achieve the required circuit response. Finally, the large sizes and the decaying performance with age of the wire-wound inductors make them undesirable for use in filters.
In a solid state filter embodiment, capacitive coupling is typically used when the application is not sensitive to the symmetry of the filter's response. When inductive elements are needed, they are typically implemented via known active circuit elements configured to simulate inductors. Like most integrated circuits, solid state inductive filters do not require constant re-tuning to maintain efficient operation. Regrettably, however, solid state circuit simulation of mutually coupled inductive circuits has been heretofore unavailable. This has limited the use of solid state filters in those applications that require mutual coupling. Accordingly, a need exists for electronically simulated mutually inductive coupling.